Sequential entangler of periodic photons in a single input and output mode

ABSTRACT

An apparatus providing an integrated waveguide device that creates entanglement between a sequence of periodically spaced (in time) photons in a single input and output mode. The invention comprises a polarization maintaining integrated waveguide chip containing a number of delay lines, integrated multimode interferometers with the potential for rapid switching, a polarization controller and off chip computer logic and timing.

PRIORITY CLAIM UNDER 35 U.S.C. §119(e)

This patent application claims the priority benefit of the filing dateof provisional application Ser. No. 61/857,710, having been filed in theUnited States Patent and Trademark Office on Jul. 24, 2013 and nowincorporated by reference herein.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

A cluster state can be loosely defined as an entangled set of qubitsarranged in a lattice. Breigel and Raussendorf strictly define a clusterstate as “Let each lattice site be specified by a d-tuple of (positiveor negative) integers a εZ^(d). Each sight has 2 d neighboring sites. Ifoccupied they interact with the qubit a”. This implies a cluster statehas interaction between all nearest neighbor qubits. In one dimension(d=1) this results in a linear chain of qubits, of arbitrary length witheach qubit entangled with both of its nearest neighbors. All of theinternal qubits will have two interactions while the edge qubits willhave one. Such a one dimensional nearest neighbor cluster state has beenshown to be amenable to several applications for computation presumingthe cluster state is “long enough”.

Traditional generation of a cluster state consists of an optical tableseveral meters on each side. On this table is a high power laser systemsuch as a pulsed Ti:Sapphire laser. The pump beam is incident on anonlinear material such as BBO, BiBO or PPKTP etc. The photons from thepump then have a small change to undergo industry standard SpontaneousNonlinear Parametric Down Conversion (SPDC) to create an entangled pairof photons, called signal and idler photons. Alternative means of photongeneration are equally valid such as but not limited to four wave mixing(FWM). To create larger cluster states the pump passes through multiplenonlinear materials (a cascade configuration) or is reflected back ontothe original material (a multi-pass configuration). These methods cancreate multiple simultaneous independent pairs of qubits. To create onelarge cluster state the pairs are sent through (i.e. acted on by) anentang operation. Normally the industry standard two qubit entanglinggate controlled phase gate (CPhase) or controlled Z gate (CZ) is used.The simplest and most efficient means of implementing the general CZgate requires 3 bulk optical asymmetric beam splitters in a specificalignment. These operations are effectively performed in parallel witheach qubit entering and exiting in its own mode. Once all the entanglingoperations are successfully completed the cluster state is fullyconstructed and an algorithm can be implemented as a sequence of singlequbit rotations and measurements on each qubit in a predeterminedsequence. Thus in the state of the art, linear cluster states arecreated from simultaneously generated qubits in parallel modes ratherthan from sequential qubits in a single mode. This is mainly due to thespontaneous nature of single photon sources. It is impossible to predictthe time between two subsequent spontaneous events.

The present invention builds upon the periodic photons source of Mowerand Englund (WO2013009946 A1) to create entanglement between sequentialseparable qubits delivered in a single mode and create a linear dusterstate of sequential qubits which is output in a single mode. Such adevice is of interest in and of itself for quantum computing.Applications include but are not limited to Measurement Based QuantumComputing (MBQC) implementation of the Detach-Joza algorithm on a fourqubit chain, arbitrary single qubit rotations on a four qubit chain,quantum key distribution, quantum information, quantum metrology andquantum lithography.

OBJECTS AND SUMMARY OF THE INVENTION

Briefly stated, the present invention (the Sequential Entangler or S.E.)combines reconfigurable optical Integrated Waveguides (IW) with aperiodic photon input to create linear cluster states in a single mode.

The present invention creates the entanglement of sequential qubits byusing a unique “loop back” architecture that delays one photon for oneperiod T of the sequence thus allowing for two photons to be acted on bya standard entangling element; which, in the present invention uses thesimple polarization encoded CZ gate of Crespi et. al (WO201215056SA1).After the CZ gate one photon (now entangled so which cannot bedistinguished) is then released and the second is looped back tocoincide with the arrival of the next photon and so on. This willprobabilistically produce a linear cluster state. The termprobabilistically as the state of the art CZ gate has a 1/9 successrate. Thus the longer a desired cluster state is the less likely it isto be created in any one attempt. This is a result of the entanglingoperation and not the S.E. per say as no photonic entangling operationcan be performed with unit success. The S.E. will create a cluster statenumerically identical to the industry standard parallel method butarranges the qubits as a periodic sequence (with a constant period T) ina single optical mode. Any two qubit entangling operation can be used inplace of the CZ however such gates may produce different cluster states.

BRIEF DESCRIPTION OF THE DRAWINGS

It is noted that all FIGURES are schematic and are said to be “not toscale”.

FIG. 1 depicts the preferred embodiment of the present invention, i.e.,full reconfigurable monolithic integrated waveguide based SequentialEntangler (S.E.) with one input mode and one output mode with labeledfeatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A sequence of periodic photons created by any means enter the integratedwaveguide chip 10 in FIG. 1 via the input port 20. The integratedwaveguide may be made from any of a number of materials. In ourpreferred embodiment we will use Lithium Niohate (LiNbO₃) as thewaveguide material. The input port 20 is a polarization maintainingoptical waveguide fabricated in the LiNbO₃ chip 10. Polarizationmaintaining waveguides are required as we chose to encode our qubits inthe polarization modes of each photon. Thus the resource of periodicphotons must also be in a known polarization state. It is then trivialto rotate the input state polarization state to any desired state via apolarization controller 30. The preferred embodiment of the presentinvention uses integrated waveguide based polarization controllers 30which function via the electro-optical effect. The preferred embodimentof the present invention utilizes the Pockets effect which is innate tolithium niohate. Such rotations could take place prior to the photonsentering the chip but for generality and controllability we rotate thepolarizations on chip. In the preferred embodiment the invention rotatesthe incoming photons at 30 to the plus state (equal superposition ofhorizontal and vertical polarization, H+V up to normalizations). Thephotons then enter the first of several multimode interferometers (MMI)40. All such interferometers are integrated on chip 10 and consist of amultimode slab of the waveguide material similar to that described bySoldano and Pennings (J. of Lightwave Tech, Vol. 13 No. 4, 1995). Theswitching and coupling effect of such MMI's 40 is depended on theirgeometry and the index change induced via the electro-optic effect. Thefabrication and operation of MMIs is well known in the state of the art.In the preferred embodiment the MMIs act as high speed spatial modeswitches that route photons from a specific input mode to a specificoutput mode. In the preferred embodiment there are several species ofMMIs (such as 1 by 2, 2 by 2 and 2 by 1 MMIs in terms of the number ofinput and output modes) however it is noted that the device could betrivially redesigned with a single species of 2×2 MMI in which unusedports are bulk terminated 50. MMI 40 switches input photons from mode 20to modes 60 or 70. In other words the MMI 40 will be controllable suchthat a photon entering in input mode 20 can be deterministically routedto either output port 60 or 70. Such MMI switches are well known in thestate of the art. The control element is therefore shown as a logicalconnection 80 and its setting is determined by chip electronics 90. Toachieve proper synchronization with the periodic input source a clocksignal 100 must be sent to the device. In the preferred embodiment thehigh speed electro-optical effect (>40 GHz) is used to modify the indexwithin each of the MMI.

One output 60 of MMI 40 is routed to a long meander delay line 110. Thelength of this meander 110 is carefully fabricated to be exactly oneperiod, T, of the periodic sequence of photons. When the first photon inthe sequence enters the chip 10 via mode 20 the first MMI 40 will routeit via 60 to the delay line 110. The second photon in the sequence willbe routed by MMI 40 to the other port 70 such that the two photons willbe synchronized in time after photon one leaves the delay 110.

Now two photons which were sequential in time are now synchronized intime on the chip 10. This allows for industry standard two qubitentangling operations to be implemented.

The photons now propagate along two parallel waveguides, the “upper” 60waveguide and the “lower” waveguide 70. Here and below “upper” and“lower” are used only in reference to the appearance of the schematicFIG. 1 and not to a design element. The photons in these modes 60 and 70then enter two parallel MMIs 120. These MMIs 120 effectively controlwhether or not the entangling operation 130 (contained in the dashedbox) is implemented. In the preferred embodiment of the presentinvention the MMIs 120 can be set to either pass the photons to theentangling operation 130 or to divert them around the entanglingoperation 130 via bypass lines 140. The preferred embodiment uses bypasslines 140 because the entangling gate 130 chosen for our preferredembodiment is the CZ gate as described by A. Crespi (WO2012150568 A1).This is a static operation thus to “turn off” the interaction thephotons must be routed around it. The length of the bypass 140 lines issuch that they are the same length as the paths in the CZ gate 130 andas such synchronization is maintained. The MMIs 120 are controlled bythe off chip electronics 90 via control lines 150. In the preferredembodiment this control is implemented similarly to that of 40. If theMMIs 120 are “on” then both photons enter the CZ gate 130 at the sametime and may become entangled. If the switches are “off” the photonsremain separable after passing through the “bypass” lines 140. Thisoperation is performed in tandem thus we refer to them as paired. Notethat the CZ gate has a success rate of one in nine (1/9) and requirestwo vacuum modes. Should the CZ gate 130 succeed or the photons bediverted to the bypass lines 140 they will then each enter another MMI.The “upper” MMI 160 will divert the photon into the loop back mode 170.The “lower” MMI 180 will divert its photon to MMI 190 and out of thecircuit via output mode 200. The MMI 160 and 180 are controlled by 90via logical control line 210. MMI 190 is controlled by 90 via logicalcontrol line 220.

The photon in the loopback mode 170 will be delayed in delay line 230.The third (and all subsequent) photons that enter the chip 10 in mode 20are rotated to the correct input polarization by 30. They are thenrouted “down” into mode 70 by MMI 40. Delay line 230 is carefullyfabricated such that the photon it holds is released at the appropriatetime such that the two photons are synchronized similar to the way thefirst two photons were synchronized. In other words, the two photonsagain reach the pair of MMI's 120 at the same time.

At this point the process repeats in that one photon is looped back andone photon is released if the CZ gate 130 succeeds. In the event thatthe CZ gate 130 fails, the desired cluster state to be created. Othermechanisms such as photon loss will also cause a failure. Such a failurecan be trivially detected via post selection by the absence of a photonfrom the sequence.

Should the chain reach the desired length the entangling operation ofthe device can be terminated by MMI 40 switching a photon into mode 60.Simultaneously the photon exiting the “loop back” can be routed out ofthe device by MMIs 120, 160 and 190. This prevents any possibleentanglement and simultaneously resets the chip 10 such that it maystart to create a new cluster state with the photon in delay 110. Recallthat the first step in building the chain was to send a photon to 110.This also gives the last photon in the previous chain time to exit thechip 10.

Having described preferred embodiments of the invention with referenceto the accompanying drawing, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

What is claimed is
 1. An apparatus for sequentially entangling photons,comprising: a wav guide chip having integrated optical components andinterconnecting optical waveguide disposed therein, wherein said opticalcomponents further comprise an input port connected to said opticalwaveguide for inputting said photons into said waveguide chip; apolarization controller connected to said input port for rotating thepolarization of said input port; a plurality of delay lines forsynchronizing said photons in time; a plurality of multimodeinterferometers for selectably routing said photons through any of saidinterconnecting optical waveguide; a controller for switching the inputmode of any of said plurality of multimode interferometers so as toroute said photons out of a selected interferometer output port; a clocksource for synchronizing said controller with the arrival of saidphotons; an entangling gate having bypass and entangling inputs andbypass and entangling outputs for selectively entangling said photons;and an output port for routing said entangled photons out of saidwaveguide chip.
 2. The apparatus of claim I. wherein said polarizationcontroller rotates the polarization of said photons to a plus state. 3.The apparatus of claim , wherein each of said plurality of multimodeinterferometers is comprised of multimode optical -waveguide material.4. The apparatus of claim 1, wherein a first of said plurality of saiddelay lines has a length chosen to provide a delay equal to the periodof the periodic sequence of said photons.
 5. The apparatus of claim 1,wherein a first of said plurality of multimode interferometerssynchronizes photons by routing a photon through a first output intosaid first of said plurality of delay lines and routing a next photonthrough a second output which bypasses said first of said plurality ofdelay lines.
 6. The apparatus of claim 1, wherein a second of saidplurality of multimode interferometers, has as a first of two inputs,the output of said first delay line.
 7. The apparatus of claim 1,wherein said second and a third of said plurality of said multimodeinterferometers are arranged in parallel, wherein said third multimodeinterferometer has as an input the non-delayed output of said firstmultimode interferometer; and wherein said second and said thirdmultimode interferometers each have a first output connected to a bypassinput and a second output connected to an entangling input of saidentangling gate.
 8. The apparatus of claim 7, wherein a fourth and afifth of said plurality of said multimode interferometers are arrangedin parallel, said fourth and fifth multimode interferometers each havinga first input connected to an entangling output of said entangling gateand a second input connected to a bypass output of said entangling gate;said fourth and fifth multimode interferometers each having a firstoutput connected to an input of a sixth of said plurality of saidmultimode interferometers; and said fourth multimode interferometerhaving a second output connected to an input of a second of saidplurality of delay lines.
 9. The apparatus of claim 8, wherein saidsecond of said plurality of said delay lines has an output connected toa second of said two inputs of said second multimode interferometer. 10.The apparatus of claim 9, wherein said sixth of said plurality of saidmultimode interferometers has an output connected to said output port ofsaid waveguide chip.